Velocity-matching seismic charge unit



May 5 1964 J. D. ElsLER ETAL 3,131,633r

vELoc1TY-MATcHING sEIsMIc CHARGE UNIT Filed nec. 1o, 1958 `2 sheets-sheet i May 5, 1964 J. D. ElsLER ETAL' 3,131,633

VELOCITY-MATCHING SEISMIC CHARGE UNIT Filed Deo. 10, 1958 2 Sheets-Sheet -2 FIG. 5 INVENTORS.

JOSEPH D. EISLER BY DANIEL SILVERMAN .maga

ATTORNEY United States Patent 3,131,633 VELOCITY-MATCHING SEISMIC CHARGE UNIT Joseph D. Eisler and Daniel Silverman, Tulsa, Okla.,

assignors to Pan American Petroleum Corporation, Tulsa, Okla., a corporation of Delaware Filed Dec. 10, 1958, Ser. No. 779,430 14 Claims. (Cl. 102-21.6)

This invention relates to seismic geophysical surveying and is directed to improvements in the explosives used for generating seismic waves. More speciiically, it is directed to an elongated charge array having, in the direction of its length, a'detonation velocity matching the seismic wave propagation velocity in the medium Where the charge is to be detonated. The term matching as used herein is to be understood as including both the usual sense of equal and also intentional deviations from equality, as when the detonation velocity of a charge is purposely made somewhat greater or less than the formation seismicwave velocity to vary the energy-directive properties of the charge.

Elongated velocity-matching explosive charges are now being used with increasing frequency for generating seismic waves in seismic geophysical exploration, due to their ability to reduce secondary or ghost reflections, ground roll, and other shot-generated noises in relation to the reected energy. The two general types of charge construction in use are (l) a continuous length or column of lowvelocity explosive and (2) an array of spaced high-velocity cartridges coupled by delay connectors. Both types of charge construction have certain advantages and disadvantages.

Theoretically, the continuous type is most free of undesirable noise generation, but it is commerically supplied only in certain discrete values of detonation velocity andpin rather large total weights of explosive. Reduction of the explosive weight per unit length is not feasible because vthe explosive material used does not detonate reliably when its cross-sectional area is less than a certain substantial size. The fixed length of this charge as manufactured may be longer or shorter than desired for conditions in any given bore hole. Variation of the detonation velocity or explosive weight distribution along the length of the continuous charge is diicult or virtually impossible.

A11 array of spaced high detonation velocity cartridges is more flexible as regards length of the array, total Weight of explosive, and variable distribution of explosive weight and velocity, but the delay connectors generally available have only certain discrete values of length and delay. Some types of such connectors also are rather inaccurate in providing their nominal time delay, and they frequently lack convenience for rapid assembly into rugged charge arrays;

-. It is a primary object of our invention to provide, forincorporation in an array of spaced charges, a novel and improved charge and/ or delay unit which avoids the foregoing noted disadvantages. A more specific object is to provide an adjustable chargel or delay unit for coupling into elongated charge arrays, with substantially complete Vflexibility as to` array length, total weight of explosive and its distribution along the array, and detonation velocity both over-all and within any part of the array. VA still further object is to provide an explosive unit which is' highly accurate as to the time delay or the eective detonation velocity provided, safe to transport and handle, inexpensive to manufacture, and easy to use and adapted `forbuilding an elongated charge array that is rugged and reliable for loading into partly blocked shot holes.

Other and further objects, uses, and advantages of the invention willbecome apparent as the description proceeds.l

3,l3l,533 Patented May 5, 1964 ICC Stated briefly, the foregoing and other objects are accomplished by an elongated tubular unit containing a flexible detonating cord of constant, high detonation velocity, and of a length suiiicient to provide a certain maximum time delay. The total cord length is substantially greater than the length of the unit. One end of the cord is movable relative to the other in such a Way that the resultant velocity provided by the unit is adjustable. This is done in one or both of two ways: (l) by varying the length of the tubular unit and thus changing the axial distance traveled by the detonation wave as it travels from end to end of the fixed length of the cord; or (2) by keeping the tubular unit length constant and varying the length of the detonating cord between the ends of the unit, thus varying the total delay time over the given interval of Vaxial length. Preferably, but knot necessarily, the ends of the detonating cord terminate in pellets of high detonation velocity explosive.

When a plurality of such units are coupled together into an elongated charge array, these small explosive pieces may constitute the main sources of energy. For greater total energy, additional explosive sticks may be inserted between the delay units, however, and these small explosive pieces kthen serve both as energy sources and as the priming or detonating means which assure the transfer of detonation between the cord and the additional sticks.

In the event the small terminal pellets of explosive are omitted from each of the charge units, then special means are necessary to obtain contact such as to insure that the propagation of detonation takes placeV between the end of the detonating cord and the charge unit to be detonated and vice versa. One embodiment of our invention accordingly provides special cord end-holding means for making such a contact and protecting it from moisture and the like which might interfere with proper transmission'of the detonation. Y

This will be better understood by reference t0 the accompanying drawings forming a part of this application and illustrating a number of typical embodiments and modifications of the invention. In these drawings,

FIGURE l is a diagrammatic earth cross-section illustrating the use of an elongated explosive charge array, with its'delay connecting units between charges shown partly in section, embodying the invention for seismic geophysical surveying;

- FIGURES 2 to` 6, inclusive, are diagrammatic longitudinal cross-sections of various modifications of a delay unit embodying the invention; v

FIGURE 7 is a diagrammatic cross-section of an explosive charge adapted for coupling to the units of the invention in assembling elongated charge arrays;

FIGURE 8 is an external view of a delay unit embodying the invention; and i FIGURE 9 is a diagrammatic cross-section View of an alternative embodiment of the invention.

Referring'now to these drawings in detail and particularly'toFIGURl'z` 1 thereof, atypical embodiment of the invention is shown in use ina shot hole for the generation of seismic Waves in seismic geophysical surveying. Thus, into a shot hole 20 extending to .a substantial depth belowthe yearths surface 21, at a location where it isdesired to generate seismic waves, is lowered a plurality ofY delay units ,22 coupling together a plurality of explosive charges 23' into an elongated explosive charge array. For the purpose ofdetonating this charge array, a detonating cap 24 in theuppermost of charges 23 is connected by, electrical leads 25 to a blaster 26 at the ground surface 21V and to a recorder 27 for recording the instant of initiating the detonation. Also connected to the recorder 27 are a plurality of seismometers 28 at the j ground surface spaced at any desired distance from each 3 other and from the upper end of the shot hole 20 in a manner well known in the art. t Y Y Further, as is well known in the art, the delay units22 and charge units 23 are so adjusted and arranged that the rate of travel of the detonation wave from the cap 24 along the length of the charge array is matched to the seismic-wave propagation velocity of the earth medium surrounding the shot hole 20. In operation, therefore, seismic waves created in the surrounding earth medium by the detonation in sequence of the various charge units 23, after transmission, reilection, and refraction in the earths subsurface, are subsequently received by the detector units 28 and recorded by the recorder 27.

As it is to the particular manner of construction of the individual delay connectors 22 that the present invention is addressed, several diirerent ways of constructing such delay units are illustrated in FIGURES 2 to 6 of the drawings. Thus, as appears in FIGURE 2, each unit 22 may comprise a length of detonating cord 30 terminating in a pair of spaced high-explosive pellets 31 and 32. The se are contained Within a tubular member made up of a pair of telescoping tubular parts 34 and 35. Suitable provision is made for inter-connecting a number of such units with each other and/or withV additional explosive sticks, such as by providing external threads 37 on the part 35 and internal threads 36 on the part 34. As is apparent from the figure, the total length of the detonating cord 30 is substantially greater than the spacing between the charges 31 and 32. The travel time of a detonation wave along the cord 30 from the charge 31 tothe charge 32, however, depends only upon the total length of the cord 30 and is independent of the spacing between the charges 31 and 32. The latter is adjustable by telescoping the parts 35 and 36, the spacing of the charge units 31 and 32 in this way determining the effec- Y tive detonation velocity across this` space. In other words, if L -is the axial length of unit 22 and T Vis the travel time of the detonation wave along the cord 30 from end to end, then, the effective axial detonation velocity V chosenv forV matching a certain seismic wave propagation velocityY is given by the simple equation The required extension of the parts 34' and 35 for vari- Vous values of the axial velocity V is preferably visually indicated in some manner, for-example, as is shown in FIGURE 8, by providing a velocity scale 41 on the outside of the part 35. Once the proper extension L of ing Vopenings 38 arepreferably provided in parts `34 Vand 35 near the ends of the cord 30.

There are a number of possible waysby which the telescoping members'34 and 35 may be Vadjusted relatively lengthwise' and maintained in place. VThus, as in FIG- URE 3, the external threads 37 may extend the entireV length of the inner member 35 and engage the internal threads 36 likewise extending throughout theY length of theV member 34. Or, as-appears in FIGURE 4, the inner Vtubular member Vmay be divided Vinto two portions 35A and 35B which` project from bothY Vends of the Youter tubular member` 34A, which is provided with both 'rightand left-handed internal threads to Yengage correspond- Ving external threads Von the VVmembers 35A and 35B respectively. Thus, by rotating only the outer sleeve 34A Y' Vwhile maintaining inner tubular members 357A and 35B stationary, changes in the effective spacing of thev charges- A 31 and 32'can be made withouttwisting theV detonating cord 3l); Q i

In all of the embodiment described above it may be noted that the possible extension of the telescoping members 34 and 35 is somewhatless than the length of either member. This accordingly limits the possible adjustment of the effective detonation velocity of the unit to a range of somewhat less than two to one. In FIGURE 5 is shown a modification of the invention which is not limited in this way. This is similar in most respects to the embodiment of FIGURE 4 with the additional provision that the individual tubular members A and 35B are of different diametei so as to telescope within each other. The rightand left-handed threads on the member 34A are of different diameters at the twoends of the member, so as to engage the external threads of the respective members 35A and 35B. As will be apparent, this construction permits an extensionof the unit to a length substantially greater than twice its length when closed and accordingly provides an increased range of adjustment `for the effective detonation velocity.

An alternative method of adjusting the elective detonation velocity between the charges 31 and 32' is illustrated in FIGURE i6. Thus, the charges 31 and 32 are placed at the ends of a non-telescoping tubular member 44 of xed length having the cord 30 extending through its interior. The total length of cord 30 is such as to provide the greatest anticipated travel time needed over a distance equal to the length of member 44. In this case, adjustment of the Yeffective velocity of detonation between the charges 31 and 32 is made by drawing out a proper length of the cord 30 through the opening 1n the center of the charge 32, the end of the cord, after it has been cut to the proper length, being then insertedV back within the charge 32 as indicated at 45. This places the exposed ends ofthe cord 30 in intimate contact with the explosive 32 as is necessary to insure proper propagation of the detonation from the cord to the charge material. External'threads 37 at each end of thetube 44 provide for connection to similar units or to additional sticks of explosive 23 by means of inter- 4 nally threaded collars 46.

In using all of the foregoing embodiments of the in- I vention the threaded coupling members are such as to permit either coupling the units directly to each other end to end or coupling to one or more supplementary explosive charges 23 like that of FIGURE 7. Thus, the latter are preferably providedV with the same type of external threads 37 and internal threads 36, on the projecting sleeve v46, as are used on the delay units 22. With such a coupling arrangement, the charges 23 can be manufactured in a single size or small variety of sizes; and any number can be connected together to make exactly the total charge weight desired'. A further advantageris that by using different weights or sizes of the supplementary charges 23Y at dierent places in an array, substantially any desired non-uniform distribution of charge weights is readily feasible. t Y

In order that small total weights ,ofV explosive can be utilized when desired, the weights or the individual pellets 31 Vand 32 should not be toolarge.V Thus, a size in the range -fro'm about one to sixteen ounces-typically about four ounces-would appear to be preferable. Such a charge size materially aids propagation of the detonationalong'and across the joints of an array but contributes only a limited amount of explosive power when such Vis needed. ,i Y v. Y A still further modification of the invention is illustrated l inFIGURE 9. Thus, as 'wasA statedV briefly above, while itis preferable that the ends of the detonating cord 30 terminate in the individual charges 31 and 32 forming apart'of kthe unit 22, these charges are not essential and Vmay be omitted-by coupling the cord 30sdirectly to the explosive cartridge, The unit will then begconstructed somewhat as shown in FIGURE 9. Thus, the ends ofV i the cord 30 areV firmly held by supporting members 51V Y and 5 2, which are respectively anchored or molde'd'inV the ends of the telescoping parts 34 and 35. Since it is essential that the ends of the cord 30 be in intimate contact with the explosive material to be detonated, the cord preferably extends outwardly through the supporting means 51 and 52 and passes through the center of a projecting rod or spike 53 thereon. Projection 53 is preferably sharpened at its end so that it may be driven into and imbedded in the interior of the explosive stick 23 to which it is connected.

In order to protect the exposed ends of the cord 30 from absorbing moisture and the like, which may interfere with its proper operation, these ends are preferably covered with a thin layer of wax or' the like for waterproofing. The members 51 and 52, including the projections 53, may be formed of any material desired but are advantageously molded of an inexpensive plastic or syntheticresin. The pointed or tapering shape of the projecting members 53 is such that it not only places the ends of the cord 30 in direct contact with the explosive material of the charge 23, but also excludes bore hole water from entering subsequently and affecting the ability f the cord and charge materials to detonate properly.

The form and nature of the detonating cord 30 are matters of some importance. Until fairly recently, the detonating cord Acommercially available under the name of Primacord contained substantial `amounts of explosive material per unit length of cord. The power of this material was such that if two pieces of the cord were in contact, detonation could be transferred from one to the other; or, if they were not in close contact but merely closely spaced, the detonation of one could sever the other and render it useless. More recently, however, detonating cords with quite small amounts of explosive per unit length have been successfully manufactured and made available commercially. With these low-energy detonatng cords, as they are known, it is no longer necessary that adjacent portions ofthe cord be physically separated by a denite spacing, since even if they are in contact, crossdetonation or cutting of one cord by the other doesV not occur. With such low-energy cords, which are preferred for use in the present invention, detonation propagation occurs only from the end of the cord, and therefore special care must be taken to insure contact of the ends of the cord with the explosive to be detonated, as has been pointed out in the above description of various embodiments of the invention where this requirement is satis- It should be noted, however, that with older forms of the detonating cord containing a higher explosive content per unit length, the objectives of theinvention can still be accomplished by keeping the convolutions of the cord Well separated, as by forming the cord into a helix or the like such that adjacent turns or portions of the cord are at least one-half inch from each other at all times. This can readily be accomplished in a number of different ways in the present invention, one being by coating the cord with a resilient synthetic resin which imparts to the helix a,v springy characteristic. Accordingly, the spacing of adjacent turns remains relatively uniform regardless of the;

length of the unit 22.

By way of example but not of limitation, a delay unit embodying the invention may employ a twenty-foot length u of the cord 30. With a detonation velocity of about 20,000 feet per second, this provides about 1 millisecond delay. If the minimum length of the unit 22 is live feet and the maximum nine feet, any resultant axial detonation velocity V from 5,000 to 9,000 feet per second can be accurately predetermined. lt will be apparent how different lengths of cord 30 and of the unit 22 can be arranged to provide the same or different values of axial velocity V.

While our invention has been described by referring to the foregoing speciic details and embodiments, it is to be understood that its scope should not be considered as limited to the details set forth. The scope of the invention,

however, should properly be ascertained by reference to the accompanying claims.

We claim:

l. An adjustable detonation-transmitting delay unit for spacing apart and operatively connecting adjacent explosive charges of an elongated charge array adapted to generate seismic waves and to match the detonation velocity along the array to the seismic-wave propagation velocity of a surrounding medium, said delay unit comprising a tubular member of a substantial length at least equal to the spacing of said adjacent charges coupled thereby, coupling means at each end of said member for connecting it into said array, a 'flexible high-detonation-velocity detonating cord extending through the interior of said member, the length of said cord being substantially greater than the minimum lengthof said member, and means for holding the ends of said cord substantially at the ends of said member, one of said cord ends being movable relative to the other in the lengthwise direction of said member to vary at least one of the variables L and T in the equation L V-r where L is the axial length of said unit, I is the travel time of a detonation wave from end to end of said unit, and V is the effective axial velocity of said detonation wave, j

which is variable until the time of connecting said unit into said array and is chosen to match the seismic wave propagation velocity of the medium wherein said array is to be detonated.

A2. A delay unit as in claim 'l in which said tubular member comprises at leastl two telescoping parts, whereby L in said equation is varied by the telescoping of said parts, and means for maintaining said parts in 'xed relative positions. Y

3. A delay unit as in claim 2 yin which said maintaining means comprises means penetrating both walls of said telescoping parts where they are in contact.

4. A delay unit as in claim 2 in which said maintaining means comprises inter-engaging threads on said telescoping parts, whereby L is varied by relative rotation of said parts.

5. A `delay unit as in claim 4 in which there are three telescoping parts, two of which telescope with the third.

6. A delay unit as in claim 4 in which there are three telescoping parts of three different diameters, whereby each telescopes with the other two.

7. A delay unit as in claim l in which said cord-endholding means comprises two blocks of 'high-detonationvelocity explosive, one at each end of said tubular member.

8. A delay unit as in claim 7 in which Iat least one of said iblocks has a central opening through which said cord may be drawn to vary its length and thereby vary T in said equation.

9. A delay unit as in cla-im l in which said oord-endholding means comprises two `axially outwardly extending projections at the ends of which said cord ends are exposed, one at each end of said tubular member.

10. A delay unit as in claim 9 wherein said tubular member comprises at least two telescoping parts, whereby L in said equation is varied by the telescoping of said parts.

ll. An adjustable detonation-transmitting delay unit for use in seismic surveying as part of an elongated array of spaced explosive charges wherein said unit is the coupling and detonation-transmitting means between each two adjacent charges of said array, said delay unit cornprising two telescoping tubular members of a combined length which is substantial and -atleast equal to the spacing of the said two adjacent charges coupled thereby, a ilexible high-detonation-velocity detonating cord extending through said tubular members substantially from end to end of said unit, a charge of high-detonation-velocity explosive enclosing each end of said cord-substantially at the ends of said unit, means at each end of said unit for coupling it into said array, and means for maintaining said telescoping members xed relative to each other with aspacing L between ends of said cord to produce an effective axial detonation velocity V matching the seismic wave propagation velocity in the medium wherein said array is to be detonated, V and L being related according to the equation where T is the travel time of a detonation wave from end to end of said cord.

12. An adjustable detonation-transmitting delay unit for use in seismic surveying as part of an elongated array of spaced charges wherein said unit is the'coupling and detonation-transmitting means between each two adjacent charges of said array, said delay unit comprising a tubular member of a substantial length which is at least equal to the spacing of the two explosive charges of said array to be coupled thereby, two cord-holding charges of highdetonation-velocity explosive each fixed at one of the two ends of said membena flexible high-detonation-velocity detonating cord extending through said member between said charges, both ends of said cord being embedded within said charges, the portion of said cord adjacent at least one of said ends extending through an opening in the corresponding cord-holding charge to the outside thereof, whereby said cord may be drawn out through said opening and cut off to vary the cord length within said member between said cord-holding charges and thereby adjust T in the equation L V n T where L is the axial length of said unit, and V is the effective axial detonation velocity chosen to' match the seismic wave propagation velocity of the'medium wherein said array is to be detonated, and T is the travel` time of a detonation wave from end to end of said cord, and means on said member for coupling it into said array.

v13. An adjustable detonation-transmitting delay unit for use in seismic surveying .asy part of an elongatedV explosive charge array formed of Va plurality of coupled members wherein said unit forms at least alternate ones of said coupled members, said delay unit comprising an elongated tubular member of substantial length, coupling-y means Iat each end' of said tubular `member for couplingl it into said array, a flexible high-,detonation-velocity detonating cord extending through the interior of said tubular CTI member, the length of said cord being substantially greater.

than the minimum length of said vtubular member, and means for holding theV endsof said cord substantially at the ends of said tubular member, one of said cord ends Vbeing movable relative t0 the other in the lengthwise direc g tion of said member to vary at least one of the Variables L and T in the equation L Vrr where L is the axial length of said unit, T is the travel time of a detonation wave from end to end of saidunit, and V is the effective axial detonation velocity of said detonation wave, which is variable until the time of connectingsaid unit into said array and is then chosen to match the seismic wave propagation velocity of the medium wherein said array is Vto be detonated.

14. A rigid connecting device of adjustable length for joining explosive charges which comprises an outer rigid elongated tube, an inner rigid elongated tube having an outside diameter slightly smaller than the inside diameter ofu said outer tube, slidably mounted and nested at least partially within said outer tube ,to form a tubular telescopingpassembly, the length of which assembly is adjustable to permit variation in the Vspacing of said explosive charges by sliding one of said tubes with respect to the other to vary the degree of nesting of the inner tube within the outer tube; a connecting means at each end of said assembly forrconnecting an explosive charge thereto, one

- of said connecting means including `a primary charge of References Cited in the tile of this patent UNITEDL STATES PATENTS 1,312,212 Saloun Aug. 5, 1919 1,493,881 .Tones etal May 13, 1924 y1,988,467 Wiley Ian. 22, 1935 2,377,151 Huber May 29, 1945 2,468,274 Riley Apr. 26, 1949 2,609,885 Silverman --.Sept 9, 1952 2,755,878 Smith July 24, 1956 2,775,200 n YGuenter Dee. 25, 1956. 2,796,834 McCalery et al June 25, 1957 2,808,894 Eisler et al Oct.y 8, 1957 2,823,609 Johnson et al. Feb. 18, 1958 2,887,053 Itria et al. May19, 1959 2,992,611 Y Felch July 18, 1961 FOREIGN PATENTS 146,238 Australia Apr. 29,1952

. Charges inra Single Seismic Shot, Geophysics Magazine,

vol. XV, No. 2, April v1950, pages 208-218. 

1. AN ADJUSTABLE DETONATION-TRANSMITTING DELAY UNIT FOR SPACING APART AND OPERATIVELY CONNECTING ADJACENT EXPLOSIVE CHARGES OF AN ELONGATED CHARGE ARRAY ADAPTED TO GENERATE SEISMIC WAVES AND TO MATCH THE DETONATION VELOCITY ALONG THE ARRAY TO THE SEISMIC-WAVE PROPOGATION VELOCITY OF A SURROUNDING MEDIUM, SAID DELAY UNIT COMPRISING A TUBULAR MEMBER OF A SUBSTANTIAL LENGTH AT LEAST EQUAL TO THE SPACING OF SAID ADJACENT CHARGES COUPLED THEREBY, COUPLING MEANS AT EACH END OF SAID MEMBER FOR CONNECTING IT INTO SAID ARRAY, A FLEXIBLE HIGH-DETONATION-VELOCITY DETONATING CORD EXTENDING THROUGH THE INTERIOR OF SAID MEMBER, THE LENGTH OF SAID CORD BEING SUBSTANTIALLY GREATER THAN THE MINIMUM LENGTH OF SAID MEMBER, AND MEANS FOR HOLDING THE ENDS OF SAID CORD SUBSTANTIALLY AT THE ENDS OF SAID MEMBER, ONE OF SAID CORD ENDS BEING MOVABLE RELATIVE TO 